There has been typically known exhaust gas recirculation in which a part of exhaust gas discharged from an engine is returned to an intake side of the engine and a combustion temperature is reduced by combusting such an intake gas with less oxygen concentration, thereby reducing NOx to be generated in the combustion.
In such exhaust gas recirculation, a recirculation pipe line communicates an exhaust manifold with an intake manifold in the engine. An exhaust gas recirculation valve is provided in the recirculation pipe line (see, for instance, Patent Literature 1). When the exhaust gas recirculation is performed, the exhaust gas recirculation valve is opened and a part of the exhaust gas is recirculated from the exhaust manifold to the intake manifold.